Measurement of subsurface formation lithology, including composition and fluids, using gamma spectroscopy and thermal neutron decay

Gamma ray spectra of earth formations surrounding an open or cased well borehole are obtained by exciting subsurface formation elements around the borehole with pulsed neutrons and detecting the gamma rays resulting from capture in the subsurface formation of thermalized neutrons from a capture gamma spectroscopy (C.G.S.) well log source. The spectra of gamma rays so obtained are analyzed to form logs of the elements which contribute significantly to the spectra. Simultaneously, the time rate of decay of the number of gamma rays resulting from capture of the thermal neutrons in the formation is obtained to derive a measure of the macroscopic thermal neutron capture cross-section of the formation. The gamma rays obtained from the formation are then compensated for variations the thermal neutron capture cross-section obtained for the formation. From the logs of elements and the macroscopic thermal neutron capture cross-section of the formation, quantitative measurements of the primary formation parameters of formation water salinity, water saturation, porosity, major matrix components and ratios of formation constituents of the earth formations are obtained